A fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemical reaction of a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air. This fuel cell is basically constructed by a pair of electrodes, namely, an anode and a cathode, formed respectively on both surfaces of a polymer electrolyte membrane that selectively transports hydrogen ions. In general, the above-mentioned electrode comprises a catalyst layer composed mainly of a carbon powder carrying a platinum group metal catalyst, and a diffusion layer which has both gas permeability and electronic conductivity and is formed on the outside surface of this catalyst layer.
In order to prevent leakage of the fuel gas and oxidant gas supplied to the electrodes and prevent mixing of two kinds of gases, gas sealing members or gaskets are arranged on the periphery of the electrodes with the polymer electrolyte membrane disposed therebetween. These sealing members or gaskets are assembled integrally with the electrodes and polymer electrolyte membrane in advance. This part is called “MEA” (electrolyte membrane-electrode assembly). Disposed outside of the MEA are conductive separator plates for mechanically securing the MEA and for electrically connecting adjacent MEAs in series, or in some cases, in parallel. Disposed at portions of the separator plates, which are in contact with the MEA, are gas flow channels for supplying reacting gases to the electrode surfaces and for removing a generated gas and excess gas. Although the gas flow channels can be provided separately from the separator plates, grooves are usually formed on the surfaces of the separator plates to serve as the gas flow channels.
In order to supply the fuel gas and oxidant gas to these grooves, it is necessary to branch pipes that supply the fuel gas and the oxidant gas, respectively, according to the number of separator plates to be used, and to use piping jigs for connecting the ends of the branched pipes directly to the grooves of the separator plates. This jig is called “manifold” and a type of manifold that directly connects the supply pipes of the fuel gas and oxidant gas to the grooves as mentioned above is called “external manifold”. There is a type of manifold, called “internal manifold”, with a more simple structure. The internal manifold is configured such that through holes are formed in the separator plates having gas flow channels and the inlet and outlet of the gas flow channels are extended to the holes so as to supply the fuel gas and oxidant gas directly from the holes.
Since the fuel cell generates heat during operation, it is necessary to cool the cell with cooling water or the like in order to keep the cell in good temperature conditions. In general, a cooling section for feeding the cooling water is provided for every one to three cells. There are a type in which the cooling section is inserted between the separator plates and a type in which a cooling water flow channel is provided in the back surface of the separator plate so as to serve as the cooling section, and the latter type is often used. The structure of a common cell stack is such that these MEAs, separator plates and cooling sections are placed in an alternate manner to form a stack of 10 to 200 cells, and this stack is sandwiched by end plates, with a current collector plate and an insulating plate disposed between the stack and each end plate, and secured with clamping bolts from both sides.
In a conventional fuel cell configuration as described above, unit cells each comprising an MEA and separator plates sandwiching the MEA are simply stacked in the thickness direction, and a fuel cell having a volume capable of being installed in mobile devices cannot be readily achieved by merely downsizing a conventional fuel cell without changing the configuration. Additionally, it is difficult to supply a fuel or oxidant gas to individual unit cells by simply stacking them in the thickness direction in order to achieve a serial connection. Further, the smaller the thickness and area of the electrode catalyst layer or polymer electrolyte membrane, the more difficult the handleability thereof during manufacturing.
In addition, the output of the fuel cell significantly depends on the surrounding environment such as temperature. Accordingly, it is difficult to install a conventional fuel cell as it stands, in mobile devices. For example, the output of the fuel cell is low at the time of start-up of the fuel cell because of a lower cell temperature compared with that at normal operation, so that it is occasionally impossible to drive a device until the cell temperature rises to a normal operating temperature. This makes it difficult to install the fuel cell in mobile devices.
It is an object of the present invention to provide a small fuel cell which employs a polymer electrolyte membrane and is suitable for use in mobile devices.
It is another object of the present invention to provide a method of manufacturing a fuel cell employing a semiconductor process.